Apparatus for correcting the transmitted signal envelope of a compatible single sideband transmitter



April 1968 T J. VAN KESSEL ET 3,378,772

APPARATUS FOR COIiRECTING THE TRANSMITTED SIGNAL ENVELOPE OF ACOMPATIBLE SINGLE SIDEBAND TRANSMITTER Original Filed April 9, 1962 2Sheets-Sheet 2 31m o u v A 9 10 Hum 27 AMPLnum. a TRANSMITTER JATKQ ATZZ26 ADDER I 5Yncnno-ou 7 Demon moR AMPLHWK: C NTROL DEVKE H -TER mo nuflmE nwoo uwk N axmmom 1-8 DEM E 8 (1-a a(|-a a 2 6-3 a fl-a a 0: P P P PmeoooRus J. VAN xssszz.

BY JOANNES M. A. UJJEN INVENTOR.

United States Patent 3,378,772 APPARATUS FOR CORRECTING THE TRANS-Ml'ITED SIGNAL ENVELOPE OF A COM- PATIBLE SINGLE SIDEBAND TRANSMITTERTheodorus Jozef van Kessel and Joannes Maria Alhertus Uijen, Emmasingel,Eindhoven, Netherlands, asslgnors to North American Philips Company,Inc., New York, N.Y., a corporation of Delaware Original applicationApr. 9, 1962, Ser. No. 186,255, new Patent No. 3,274,492, dated Sept.20, 1966. Divided and this application July 21, 1966, Ser. No. 566,863Claims priority, application Netherlands, May 16, 1961, 264,824 Claims.(Cl. 325-137) This application is a division of application Ser. No.186,255, filed Apr. 9, 1962, and now Patent Number 3,274,492, grantedSept. 20, 1966.

This invention relates to transmitters for the transmission ofamplitude-modulated oscillations, wherein the information content isconcentrated substantially on a single side-band. In transmitters ofthis type, carrier oscillations and the signals to be transmitted areapplied to an amplitude modulator. The carrier oscillation and onesideband are derived from the modulator, by means of an output filter,for further transmission.

Transmitters of the above kind afford important advantages from aviewpoint of transmission technique, since in the first place theamplitude of the emitted information signals may be considerablyincreased with respect to the amplitude of the carrier wave unvariedpower of the transmitter. More particularly with normal amplitudemodulation the amplitude of each side-band is half the amplitude of thecarrier wave with maximum modulation. When only one side-band istransmitted the amplitude of the side-band signal may be equalized tothe amplitude of the carrier wave. In addition, a saving in bandwidth isobtained so that the frequency space between transmitters havingadjacent frequency bands may be reduced and interaction of suchtransmitters may be greatly reduced.

An object of the invention is to provide a transmitting device of theabove described type in which the reception of signals emitted by thetransmitting device in an ordinary amplitude-modulation receiverresultsin an excellent quality of reproduction while retaining theaforementioned advantages.

According to the invention the first-mentioned amplitude modulator isfollowed by a second amplitude modulator in which the signal derivedfrom the first-mentioned amplitude modulator as a carrier oscillation isamplitude modulated by the same signal as a modulating signal. Thetransmitter also comprises an output filter which passes only thesignals located in the signal band at twice the carrier frequency.

In order that the invention may be readily carried into effect, it willnow be described in detail, by way of example, with reference to theaccompanying diagrammatic drawings, in which:

FIGURE 1 is a block diagram of a transmitter according to the invention;

FIGURES 2a-2h show several diagrams to explain the operation of thetransmitter of FIGURE 1;

FIGURE 3 shows the transmitter of FIGURE 1 in greater detail;

FIGURE 4 shows another embodiment of a transmitter according to theinvention in which the reproduction quality upon reception in anordinary amplitude-modulation receiver is improved;

FIGURES 5a and 5b show several frequency diagrams to explain theoperation of the transmitter of FIGURE 4;

FIGURE 6 shows a preferred embodiment of a transmitter according to theinvention with improved quality of reproduction, and

FIGURE 7 shows a frequency diagram to explain the operation of thetransmitter of FIGURE 6.

The transmitter shown in FIGURE 1 is adapted for the transmission ofspeech signals in the band from 300 c./s. to 3,400 c./s. Signalsoriginating from a microphone 1 are applied by way of a low-pass filter2 and a low-frequency amplifier 3 to a modulator stage 4 including acarrier-wave oscillator 5 of, for example, 400 kc./s. The output of themodulator 4 is applied to an output filter 6 which passes, for example,the upper side-band located in the band from 400.3 kc./s. to 403.4kc./s. and the carrier oscillation of 400 kc./s. The output signal offilter 6 is applied to a device to be described hereinafter. Then it isamplified and, if desired, transposed in frequency in a transmittingstage 7, and transmitted by means of an antenna 8.

If the transmitting device shown in FIGURE 1 transmits a singlesinusoidal speech oscillation of angular frequency p, a speech side-bandw+p occurs at the output of the modulator stage in addition to theemitted carrier oscillation T of angular frequency 0:. Theseoscillations are shown in a frequency diagram in FIGURE 20:, in whichthe amplitude of the carrier oscillation is reduced to unity and thespeech side-band has an amplitude a which must be smaller than theamplitude 1 of the carrier-wave in order to prevent overmodulation.

As previously mentioned, the transmitter under consideration has, withrespect to ordinary amplitude-modulation transmitters, the advantagethat the amplitude of the information signals may be considerablyincreased with unchanged power of the transmitter and that an increasein efficiency is thus obtained. This is offset 'by the fact thatconsiderable signal distortion occurs upon reception of the signalsemitted by the transmiter in an ordinary amplitude-modulation reeciver,as will be explained more fully with reference to the vector diagramshown in FIG. 2b.

In this figure, the vector T again shows the carrier oscillation aboutwhich the sinusoidal oscillation a rotates at angular frequency p. Thesum vector E of these vectors describes the envelope of the emittedoscillations, which, as may appear from the time diagram shown in FIGURE20, no longer varies sinusoidally so that, upon detection of thisenvelope in an ordinary amplitude-modulation receiver, distortionproducts occur in addition to the desired speech frequency p.

More particularly, this envelope is represented mathematically by theformula:

E= /(1+a cos pt) +a S1112 pt or developed in a series:

Upon transmission of the frequency spectrum shown in FIGURE 2a, thereoccurs at the output of an ordinary amplitude-modulation receiver, inaddition to the desired speech-frequency component a cos pt, thedistortion products cos 2pt, cos 3p! etc., which increase progressivelywith increasing amplitude a of the speech component. For example, it thespeech component has a maxim-um amplitude 0 equal to 0.7 times theamplitude of the carrier wave, the total distortion level is 17%relative to the carrier level, corresponding to 15 db.

To illustrate this, FIGURE 2d shows the frequency spectrum which occursat the output of the amplitudemodulation receiver, the values of thevarious frequency components also being indicated. If the speech signalcomprises a plurality of frequency components, for example if theangular frequency component q is present in addiLion to the frequencycomponent 2, interference products of frequencies p-l-q, pq, Zp-l-q, 2pqetc. occur in addition to the harmonics of each of the components p andq. The total distortion level composed of the level of the harmonicdistortion components 2p, 2q, etc. and the level of the interferenceproducts p-l-q, pq etc. relative to the carrier level is a l4 db for amodulation depth of 0.7 and equal levels of the speech components p andq.

An object of the invention is to provide a transmitting device in whichthe reproduction quality upon reception in an ordinaryamplitude-modulation receiver is considerably improved while retainingthe above-mentioned advantages. According to the invention, this objectis attained by providing a second amplitude modulator 9 in which thesignal derived from the first-mentioned amplitude modulator 4 as acarrier oscillation is modulated in amplitude by the same signal as amodulating signal. The transmitting device also includes an outputfilter 10 which passes only the signals located in the signal band attwice the carrier frequency. In the embodiment shown in FIG. 1, thesignal derived from amplitude modulator 4 is applied, on the one hand,through conductor 11 as a carrier oscillation and, on the other hand,through conductor 12 as a modulating signal.

For example, in the embodiment shown, as a result of the amplitudemodulation, the signals shown in FIG- URE 2a are transmitted. Thesesignals comprise the carrier wave T of amplitude 1 and frequency w,together with the speech frequency side-band of amplitude a andfrequency w-t-p. This signal, given by the formula cos wt-I-a cos(w-l-p)! is modulated by itself in the amplitude modulator, resulting ina signal /2cos Zwt-l-a cos (2w-l-p)z-}- /2a cos (2w]2[))t of twice thecarrier frequency. This signal is passed by output filter 10. FIG. 22shows the transmitted signals in a frequency diagram in which theamplitude of the carrier oscillation having a frequency 2w is againreduced to unity.

By the use of the steps according to the invention, an additionalfrequency component of amplitude 12 and frequency 2w+2p is transmittedtogether with the carrier oscillation of frequency 2m and amplitude 1and the speech side-band of amplitude 2a and frequency 2w+p (see FIGURE2c), resulting in signal distortion being reduced to a high degree uponreception of the transmitted oscillations in an ordinaryamplitude-modulation receiver. In fact, if the envelope of thetransmitted oscillations is calculated in the manner describedhereinbefore, the envelope is represented mathematically by the formula:

/(l+2a cos pt-l-a cos 2pt) (2a sin pif-l-a sin Zpt) which afterconversion gives:

1+a +2a cos pt that is to say no distortion occurs upon reception in anordinary amplitude-modulation receiver. FIGURE 2f shows the frequencyspectrum of the envelope signal for the sake of completeness.

Thus, in order to reduce signal distortion upon reception with anordinary amplitude-modulation receiver, the transmitter according to theinvention transmits, together with the speech side-band of frequency2w+p and amplitude 2a, a correction side-band of frequency 2w+2p andamplitude a the magnitude of which, when starting from a maximummodulation depth to smaller modulation depths, will decreaseprogressively because of its quadratic character. For example, theamplitude of the correction term is 0.12 for a maximum modulation depthof 0.7, at which a is 0.35, whereas this amplitude is not more than 0.04for a modulation depth of 0.4. Due to the fact that the amplitude of thecomponents of the frequency spectrum greatly decreases towards thehigher signal frequencies with speech signals or with music, thecorrection terms greatly decrease for these higher signal frequenciesand may be suppressed for the higher signal frequencies located outsidethe signal band without substantially affecting the reproduction qualityso that in a transmitter according to the invention the bandwidth of thesignal may be maintained without disadvantage for the reproductionquality being involved.

The behavior of the above described transmitter if a plurality offrequency components are simultaneously present in the signal band, forexample if in addition to the speech frequency [1 of amplitude a anotherspeech frequency q of amplilude b is transmitted, will now be consideredwith reference to the frequency diagrams shown in FIGURES 2g, 211 and21'. FIGURE 2g shows the frequency location of the frequency componentsp and q in the output circuit of amplitude modulator 4, and FIGURES 211and 21' show respectively the associated frequency diagrams of thesignals transmitted by the transmitter and of the associated envelopesignal, which may be calculated in the manner as explained hereinbefore.

As may appear from FIGURE 2h, in addition to the speech side-bands 2w+pand 2w+q of amplitudes 2a and 2b, the frequency components Zw-l-Zp,2w+p+q and 2w+2q of amplitudes a 2ab and 11 are transmitted. In thecorresponding frequency diagram of the envelope signal in FIGURE 21, asbefore, no harmonic distortion products are present, and only a singlecomparatively small intermodulation product q of amplitude Zab occurs.The level of this product as compared with the level of theintermodulation products in a known device, is further considerablyimproved. For example, when assuming the theoretically most unfavorablecase of maximum modulation depth of say 0.7, that is to say that the sumof the speech components 2a and 2b is 0.7 and of equal amplitudes 2a and2b for the speech components p and q, then the distortion levelcharacterized by this intermodulation term relative to the carrier levelis about 0.06, corresponding to a distortion level of 25 db. As a matterof fact, the mean distortion level is considerably lower than thistheoretical maximum which can occur only in very special conditions.

When the signals transmitted by a transmitter according to the inventionare received in an ordinary amplitude-modulation receiver, only a singleintermodulation term remains of all the distortion products, asexplained in FIGURE 21'. The total distortion level is reduced to atmost 25 db, which is permissible for small transmitters for speechsignals, for example mobilephone apparatus, but which is still too highfor broadcast purposes which require a distortion level of at least 40db. FIGURES 4 and 6 show further embodiments of the decive according tothe invention in which the quality requirements for broadcast purposesare fulfilled. Before explaining this device more fully, a detaileddescription of the device shown in FIGURE 3, which is particularlyadvantageous in practice, will first be given.

FIGURE 3 shows modulator stage 9 in greater detail, the other elementsbeing identical with those in FIGURE 1 being indicated by the samereference numerals.

In the device shown in FIGURE 3 the modulator stage 9 is formed by amultigrid tube in the form of a heptode 13 in which the output signalfrom modulator stage 4 is applied to the first and also to the thirdgrid. The signals applied to the first and third grids are modulated inthe multigrid tube, the signals located in the signal band beingselected at twice the carrier frequency for further use in thetransmitting apparatus by means of a bandpass filter 10 included in theanode circuit of heptode 13.

The described modulating stage has, for the specified purpose, theimportant advantage that modulation may be effected withoutamplification of energy, and unwanted modulation products may be greatlyreduced by applying the signals to the third grid in phase opposition tothe signals applied to the first grid, which may be achieved in thedevice shown by using a pentode 14 connected as an amplifier.

FIGURE 4 shows a block diagram of a transmitter according to theinvention in which the quality requirements for broadcast purposes arefulfilled, that is to say the distortion level is more than 46 db belowthe desired signal upon reception in an ordinary amplitude-modulationreceiver. As in the device decribed with reference to FIGURE 3, elementsidentical With those of FIGURE 1 are indicated by the same referencenumerals.

In the described device in which as previously explained in detail withreference to FIGURES 1 and 2, a frequency spectrum of the emittedsignals, together with an associated envelope signal are obtained, whichform already a first approximation for realizing the qualityrequirements for broadcast purposes, the steps for completely satisfyingthe said quality requirements have the character of a correction on theemitted frequency spectrum and the associated envelope signal. Due tothese steps for realizing the quality requirements, the resultingadvantages, namely the increase in amplitude of the transmittedinformation signals and the decreased interaction of frequency-adjacenttransmitters, are fully retained.

In the device shown in FIGURE 4, for this purpose the distortionproducts occurring in an ordinary amplitude detector upon detection ofthe output signals from amplitude-eomparison device 15 which iscontrolled, on one hand, by the output voltage of an amplitude detector16 connected to the output circuit of amplitude modulator 9 and, on theother hand, through conductor 17 by the low-frequency signals to betransmitted. The distortion products, after being modulated in apushpull modulator 18, are subseqneutly transmitted by antenna 8 as acompensation term in the correct phase and amplitude, together with thesignals from amplitude modulator 9, on a carrier frequency 24corresponding to the transmitted signals. The low-frequency signals tobe transmitted are obtained by demodulation of the output signals fromamplitude modulator 4 in a synchronous demodulator 19 and an associatedlow-pass filter 20 to which the carrier oscillation from oscillator 5 issupplied for demodulation, the conductor 17 to the amplitudecomparisondevice 15 including an adjustable amplitudecontrol device 21 and anadjustable phase-shifting network 22 which, serve for adjustment of thecorrect amplitude and phase in comparing the amplitudes. If desired, thelow-frequency signals to be tranmitted may be derived directly fromlow-frequency amplifier 3, but deriving the speech signals bydemodulation of the output signals from amplitude modulator 4 insynchronous demodulator 19 has been found advantageous in view of thecorrect phase relation obtained during comparison of the amplitudes.

The output signals from amplitude-comparison device 15 are modulated,with suppression of the carrier Wave, in push-pull modulator 1S and anoutput filter 23 on a carrier frequency 2w corresponding to thetransmitted signals and which is obtained by frequency-doubling of thefrequency of carrier-wave oscillator 5 in a frequency doubler 24, theresulting side-bands being combined through an adjustableamplitude-control device 25 and an adjustable phase-shifting network 26with the output signals from amplitude modulator 9 in an adding device27 and emitted by antenna 8. With correct adjustment of the amplitudeand phase, the two side-bands upon detection in an ordinary amplitudedetector provide a correction term which is equal in value, but in phaseopposition to the distortion products occurring upon amplitude detectionof the output signals from amplitude modulator 9, so that the qualityrequirements for broadcast purposes are fulfilled, for example thedistortion level has been reduced to 46 db.

The frequency diagram of the signals emitted by the transmitter ofFIGURE 4, together with that of the associated envelope signal, will nowbe explained with reference to FIGURE 5.

As previously explained in the foregoing, upon transmission of a singlespeech frequency p, distortion products do not occur at the output ofamplitude detector 16 so that the output voltages of amplitude detector16 and synchronous demodulator 19 balance each other inamplitude-comparison device 15, so that no signal is applied to addingdevice 27 through push-pull modulator 18. The transmitted frequencyspectrum then corresponds to FIG- URE 2e and that of the associatedenvelope signal to FIG. 2

Upon transmission of a plurality of speech frequencies, for example ifin addition to speech frequency p, a second speech frequency q istransmitted, a distortion product of frequency q-p occurs, as shown inFIGURE 2i, in the output circuit of amplitude detector 16 and ismodulated via amplitude-comparison device 15 in push-pull modulator 18,with suppression of the carrier wave, on the carrier frequency 20: sothat two side-band frequencies 2w(q-p) and 2w+(q-p) located one on eachside of the carrier frequency are supplied to adding device 27.

FIGURE 5a shows the frequency spectrum transmitted by the transmitter ofFIGURE 4-, in which, in addition to the frequency spectrum emitted byamplitude modulator 9 (see FIGURE 211), two further side-bandfrequencies 2w(qp-) and Zw-l-(q-ri) of amplitude ab located one on eachside of the carrier oscillations are transmitted. The transmittedfrequency spectrum then lies substantially on one side of the carrieroscillation 2w.

FIGURE 5b shows the frequency spectrum of the envelope signal in whichdistortion upon reception in an ordinary amplitude-modulation receiveris substantially compensated due to the cotransmitted side-bandfrequencies 2w(qp) and 2w+(qp). For this purpose it is not essential totransmit both side-bands 2w-(q-p) and 2w-1-(g-p), it being possible tosuppress the side-band Zw-(q-p) by means of a filter, in which event theother side-band 2w+(qp) must be increased twice in amplitude, sinceamplitude detection of this side-band by the carrier wave 2w againprovides the desired compensation term of frequency (q-p) and amplitude2ab for the distortion products occurring upon amplitude detection. Inthis case the transmitted signal has the character of a pure singleside-band.

In the described device the transmission quality suitable for broadcastpurposes is obtained by means of a compensating device controlled by thetransmitted lowfrequency signals, which device either brings theenvelope of the signals transmitted by amplitude modulator 9 in its formpractically in conformity with the low-frequency signal to betransmitted, or renews the transmitted envelope signal. The advantageswith regard to the increased amplitude of the transmitted informationsignals and decreased infiuencing of frequency-adjacent transmitters areretained because of the corrective character of the steps carried out.

FIGURE 6 shows a further embodiment of a transmitter according to theinvention for obtaining a reproduction quality suitable for broadcastpurposes, which affords particular advantages in practice since itpermits the use of high-power modulation. As in the device shown inFIGURE 4, for this purpose the envelope signal of the signalstransmitted byamplitude modulator 9 is renewed, but in this case theenvelope is renewed in a different way, namely by replacing the envelopeof the output signal from amplitude modulator by the envelope of theoriginal low-frequency signal.

More particularly in this device the output signal from amplitudemodulator 9 is limited to a constant value in an amplitude limiter 23,the limited signal of constant amplitude, after amplification in anamplifier 29, being applied as a carrier oscillation to a high-powermodulator 30. The signal obtained by limitation is a phase-modulatedsignal the frequency spectrum of which may be mathematically calculatedin a comparatively simple manner, that is to say modulation of thissignal by the associated envelope signal must provide again thefrequency spectrum transmitted by amplitude modulator 9.

To illustrate this, FIGURE 7 shows the frequency spectrum of the limitedsignal on the transmitted carrier frequency 2w if only one speechcomponent of frequency p and amplitude a is transmitted.

In the device shown in FIGURE 6, the envelope of the signals transmittedby amplitude modulator 9 is replaced by the low-frequency signal to betransmitted, which is derived from synchronous demodulator 19 in themanner previously described with reference to FIGURE 4 and appliedthrough an adjustable amplitude-control device 31 and an adjustablephase-shifting network 32, after amplification in an amplifier 33, as amodulating signal to high-power modulator 30. The signal thus modulatedin amplitude in the high-power modulator is transmitted through anoutput network 34 by antenna 8.

As in the device shown in FIGURE 4, the envelope of the signaltransmitted by amplitude modulator 9 is renewed, FIGURE 2f showing thefrequency spectrum of the envelope signal upon transmission of onespeech frequency and FIGURE 5b showing the frequency spectrum upontransmission of a plurality of speech components, for example if speechfrequency q is transmitted simultaneously with speech frequency p. Thedevice of FIG- URE 6 permits reception that is theoretically free .ofdistortion in an ordinary amplitude-modulation receiver.

The corresponding frequency spectra of the signals emitted by thetransmitter of FIGURE 6 correspond to the frequency spectra shown inFIGURES 2e and 5a.

In this connection it is to be noted that existing amplitude-modulationtransmitters may be rebuilt in a simple manner to form a transmittingdevice according to the invention by using the steps described withreference to FIGURE 6 whereby with unchanged power of the transmitterthe amplitude of the transmitted information signals is increased andthe influence of frequency-adjacent transmitters is decreased.

What is claimed is:

1. A transmitter for transmitting single sideband signals comprising asource of information signals, a source of oscillations of predeterminedfrequency, means for amplitude modulating said oscillations with saidinformation signals to provide first single sideband signals includingsaid oscillations and one modulated-sideband thereof, an amplitudemodulator, first and second paths for applying said first singlesideband signal to said amplitude modulator, said amplitude modulatorcomprising means for multiplying the signals applied by way of saidfirst and second paths, whereby said amplitude modulator produces asecond single sideband signal in a frequency band at twice the frequencyof said oscillations, said second single sideband signal including acarrier oscillation at twice the frequency of said oscillations, meansfor synchronously demodulating a portion of said first single sidebandsignal, means for detecting a portion of said second single sidebandsignal, means for comparing the outputs of said demodulating anddetecting means to provide a correction signal, means for correcting theenvelope of said second single sideband signals with said correctionsignal, and means for transmitting said envelope corrected second singlesideband signal.

2. A transmitter for transmitting single sideband signals comprising asource of information signals, a source of oscillations of predeterminedfrequency, means for amplitude modulating said oscillations with saidinformation signals to provide first single sideband signals includingsaid oscillations and one modulated sideband thereof, an amplitudemodulator, first and second paths connected to said amplitude modulatormeans, means for applying said first single sideband signal to saidfirst and second signal paths, said amplitude modulator comprising meansfor multiplying the signals applied thereto by way of said first andsecond paths, whereby said amplitude modulator produces a second singlesideband signal in a frequency band at twice the frequency of saidoscillations, said second single sideband signal including a carrieroscillation at twice the frequency of said oscillations, means forsynchronously demodulating a portion of said first single sidebandsignal, means for detecting a portion of said second single sidebandsignal, means for comparing the outputs of said demodulating anddetecting means to provide a correction signal, means for frequencydoubling said oscillations, means for modulating said doubledoscillations with said correction signal, means for adding saidmodulated doubled oscillations and said second single sideband signals,and means for transmitting the output of said adding means.

3. The transmitter of claim 2, comprising amplitude and phase correctingmeans connected between said synchronous demodulating means and saidcomparing means.

4. The transmitter of claim 2, comprising amplitude and phase correctingmeans connected between the output of said means for modulating saiddoubled oscillations and said adding means.

5. The transmitter of claim 2, in which said means for modulating saiddoubled oscillations is a push-pull modulator.

References Cited UNITED STATES PATENTS 2,989,707 6/1961 Kahn 332--45ROBERT L. GRIFFIN, Primary Examiner.

B. V. SAFOUREK, Assistant Examiner.

1. A TRANSMITTER FOR TRANSMITTING SINGLE SIDEBAND SIGNALS COMPRISING ASOURCE OF INFORMATION SIGNALS, A SOURCE OF OSCILLATIONS OF PREDETERMINEDFREQUENCY, MEANS FOR AMPLITUDE MODULATING SAID OSCILLATIONS WITH SAIDINFORMATION SIGNALS TO PROVIDE FIRST SINGLE SIDEBAND SIGNALS INCLUDINGSAID OSCILLATIONS AND ONE MODULATED SIDEBAND THEREOF, AN AMPLITUDEMODULATOR, FIRST AND SECOND PATHS FOR APPLYING SAID FIRST SINGLESIDEBAND SIGN AL TO SAID AMPLITUDE MODULATOR, SAID AMPLITUDE MODULATORCOMPRISING MEANS FOR MULTIPLYING THE SIGNALS APPLIED BY WAY OF SAIDFIRST AND SECOND PATHS, WHEREBY SAID AMPLITUDE MODULATOR PRODUCES ASECOND SINGLE SIDEBAND SIGNAL IN A FREQUENCY BAND AT TWICE THE FREQUENCYOF SAID OSCILLATIONS, SAID SECOND SINGLE SIDEBAND SIGNAL INCLUDING ACARRIER OSCILLATION AT TWICE THE FREQUENCY OF SAID OSCILLATIONS, MEANSFOR SYNCHRONOUSLY DEMODULATING A PORTION OF SAID FIRST SINGLE SIDEBANDSIGNAL, MEANS FOR DETECTING A PORTION OF SAID SECOND SINGLE SIDEBANDSIGNAL, MEANS FOR COMPARING THE OUTPUTS OF SAID DEMODULATING ANDDETECTING MEANS TO PROVIDE A CORRECTION SIGNAL, MEANS FOR CORRECTING THEENVELOPE OF SAID SECOND SINGLE SIDEBAND SIGNALS WITH SAID CORRECTIONSIGNAL, AND MEANS FOR TRANSMITTING SAID ENVELOPE CORRECTED SECOND SINGLESIDEBAND SIGNAL.